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Dialog units are Windows' standard way of measuring distance on screen.
Dialog units are defined as one-quarter the average character width by one-eighth the average character height of the default font used on a window (creating a roughly square unit of measurement). The actual size of a dialog unit can vary from one window to the next, depending upon your selection of the font to use for the window. If you do not choose a specific font for a window, the Windows default font is used as the default font for the window and as the basis for the definition of the size of a dialog unit.
The purpose of using such a “floating definition” for sizing and positioning controls in Windows is to ensure that the relative design of your windows is kept despite any font change. The user can change their Windows default font in SYSTEM.INI, and Windows video drivers supplied by video board manufacturers can also change the Windows default font. Therefore, using dialog units ensures consistent relative window and control size and positioning from one computer to the next.